Geoengineering could bring severe drought to the tropics, research shows

Study models impact on global rainfall when artificial volcanic eruptions are created in a bid to reverse climate change
Damian Carrington theguardian.com 8 Jan 14;

Reversing climate change via huge artificial volcanic eruptions could bring severe droughts to large regions of the tropics, according to new scientific research.

The controversial idea of geoengineering – deliberately changing the Earth's climate – is being seriously discussed as a last-ditch way of avoiding dangerous global warming if efforts to slash greenhouse gas emissions fail.

But the new work shows that a leading contender – pumping sulphate particles into the stratosphere to block sunlight – could have side-effects just as serious as the effects of warming itself. Furthermore, the impacts would be different around the world, raising the prospect of conflicts between nations that might benefit and those suffering more damage.

"There are a lot of issues regarding governance – who controls the thermostat – because the impacts of geoengineering will not be uniform everywhere," said Dr Andrew Charlton-Perez, at the University of Reading and a member of the research team.

The study, published in the journal Environmental Research Letters, is the first to convincingly model what happens to rainfall if sulphates were deployed on a huge scale.

While the computer models showed that big temperature rises could be completely avoided, it also showed cuts in rain of up to one-third in South America, Asia and Africa. The consequent droughts would affect billions of people and also fragile tropical rainforests that act as a major store of carbon. "We would see changes happening so quickly that there would be little time for people to adapt," said Charlton-Perez.

Another member of the research team, Professor Ellie Highwood, said: "On the evidence of this research, stratospheric aerosol geoengineering is not providing world leaders with any easy answers to the problem of climate change."

The study considered what would happen if carbon dioxide levels quadrupled in the atmosphere – the sort of extreme situation in which geoengineering might be seriously considered. Without intervention, temperatures rose by 4C, far above the 2C level considered dangerous by the world's governments.

But the temperature rise was reduced to zero if a massive geoengineering effort took place. The 60m tonnes of sulphur dioxide pumped into the stratosphere each year in the simulation is equivalent to five volcanic eruptions, each on the scale of Mount Pinatubo, the huge 1991 eruption in the Philippines that cut global temperatures by about 0.5C in the following year or two.

The sulphate particles in the model not only reflected incoming sunlight, cutting temperatures, but also absorbed heat rising up from the Earth's surface. This reduced the temperature difference between the lower and upper atmosphere, which is the engine that drives cloud formation and rainfall. The reduction in rainfall seen in the geoengineering model was as big as the increase in rainfall projected if global warming was unabated.

Dr Matthew Watson, a researcher at the University of Bristol and advocate of further research into geoengineering, said: "The researchers chose an extreme climate scenario so we should not be surprised if that, and any geoengineering attempt to counter it, had severe and uneven impacts."

He added: "It remains the case that our only guaranteed way forward is to reduce the record levels of greenhouse gases we continue to pump into the atmosphere. It's vital that scientists continue researching geoengineering; but no government serious about climate change should see it as a quick fix."